In a packet scheduling based communication system, a first communication station represented by a base station (BS) and a second communication station represented by a UE transmit and receive data through mutual wireless connection. The BS and the UE confirm mutual downlink and uplink channel status and selectively apply any transmission mode, thereby raising the efficiency of the communication system.
In conventional code-division multiple access (CDMA), which is based on packet scheduling, and orthogonal frequency division multiplexing (OFDM), which has been widely considered recently, a method of exchanging channel status of a user to give communication priority to UEs having a good channel status is mainly used. When multiple users are present, a channel status of each user should be recognized as accurately as possible to obtain multi-user diversity. Since OFDM transmits signals using subcarriers, channel variation in frequency as well as channel variation in time should be accurately transmitted to maximize the use efficiency of a frequency channel.
To generate channel information of a user, a method of simply generating average quality of a reception signal over an entire channel bandwidth and coding the quality, or a method of applying differential encoding using correlation with a previous transmission value, may be used.
The digital channel information generated by the aforementioned method is transmitted to a transmitting side. The transmitting side selects a user having a good channel status based on the channel information and generates traffic at a next transmission time interval (TTI). The traffic is modified to transmit values for an entire frequency bandwidth in OFDM, and the values are not expressed as one value for the entire frequency bandwidth but are discriminated in prescribed units of frequency bandwidths. Qualities of reception signals of the discriminated units may be calculated and encoded. Alternatively, differential encoding may be performed using correlation between the qualities or correlation with a previously transmitted value. Thus the generated channel quality information (CQI) or channel state information (CSI) is transmitted to the transmitting side and the transmitting side selects users having a good channel status in a frequency region of the next TTI.
The generated CSI is transmitted from a receiving side to the transmitting side, and a transmission scheme differs according to which side transmission of the CSI start. A general transmission scheme is divided into event-triggered CSI reporting and periodic or on-demand CSI reporting. In event-triggered CSI reporting, a user senses variation in a channel status and transmits CSI to the transmitting side. The periodic CSI reporting serves to transmit CSI of a user through a resource location determined at regular intervals. The on-demand CSI reporting commands the receiving side to transmit CSI at a time point demanded by the transmitting side.
In a conventional event-triggered CSI transmission scheme, CSI reporting is started from a BS to UE. Although in the event-triggered CSI transmission scheme the receiving side needs to feed back CSI according to variation in an initial state of a UE and channel environments, such an operation has yet to be implemented. Furthermore, a control channel can not be used efficiently.